The present invention relates to wafer holders and more particularly to wafer holders suitable for use in semiconductor wafer processing such as the electron beam exposure of wafers.
Semiconductor processing requires the wafer to undergo many different steps such as oxidation, gaseous deposition, ion implantation, metal deposition, electron beam exposure and gaseous etching. These steps are carried out by using a wafer holder suitable for each step.
The electron beam processing step is one which is usually more accurate than other types of photolithograph. In a typical example of electron beam processing, the wafer is comprised of doped silicon covered by an insulating layer. The wafer surface is coated with an electron beam resist. The wafer is mounted in a wafer holder and placed in an electron beam exposure system.
In a typical exposure system, the wafer in a holder is placed on a table movable in X- and Y- axes directions with the aid of servomotors. The servomotors are controlled by a computer.
The scanning area of the electron beam typically is restricted to a movement of 0.5 millimeters and thus the movement is much smaller than the typical chip area. In such an environment, it is necessary to rigidly hold the wafer and wafer holder relative to the moving table.
Several techniques have been utilized to hold the wafer holder and the wafer. One technique employs a magnetic chuck in which a magnetic metal holder is rigidly held by a magnetic field. This method is not desirable, however, because the magnetic field affects the position of the otherwise accurately positioned electron beam. Another technique employs a vacuum chuck in which the wafer and wafer holder are held in place by a vacuum. This vacuum technique, however, is not readily useable in electron beam systems since the work chamber in the electron beam system itself must be maintained as a vacuum. Because of the problems of magnetic and vacuum techniques, mechanical techniques are more desirable.
While a holder having a leaf spring for holding the wafer against mechanical stops on the wafer holder has been attempted, some difficulties have arisen from this and other mechanical techniques. For instance, relatively small sized wafers were too loose so that the wafers either moved or fell out of the wafer holder. On the other hand, if the spring force was increased to hold the wafers, relatively large-sized wafers tended to have broken edges as a result of the spring force.
In view of the above background, an object of the present invention is to provide an improved mechanical wafer holder which holds the wafer rigidly without damage and is suitable for use in electron beam and other apparatus employed to process semiconductors.
Another object of the present invention is to provide a wafer holder capable of sustaining the wafer rigidly.
Additional objects and features of the invention will appear from the following description in which the preferred embodiments of the invention have been set forth in detail in conjunction with the drawings.